[PokéYellow] Questions about Pikachu’s Evolution

I’ve been tampering with Pikachu’s evolution lock lately, and I came to the decision that I want Pikachu to evolve by Level-Up at Lv20, but I still want the starter Pikachu to be locked out of the new level up evolution. I know this has to do with engine/items/items.asm, but how can I get this to work?

Caramba! I hate it when there’s a lot of hair on the floor! Here, I’ll go clean up!Woo! Feels good when the floor is clean, doesn’t it? Let’s groove.